RITCHIE, WILLIAM, LL.D., an ingenious self-taught philosopher, was originally educated for the Church of Scotland, is which he was licensed to preach the gospel. He became rector of the Royal Academy of Tain, in Ross-shire, where he contrived, by extreme frugality, to save from his small annual stipend a sum sufficient to enable him to attend a course of the lectures of Messrs. Thenard, Gay-Lussac, and Biot, at Paris, and also to pay a substitute for the performance of his duties during his temporary absence from Scotland. His skill and originality in devising and performing experiments with the most simple materials, in illustration of various disputed points of natural philosophy, attracted the attention of the celebrated philosophers whose occasional pupil he had become. He had also communicated to the Royal Society, through Sir John Herschell, who took a strong interest in his fortunes, papers ‘On a New Photometer;’ ‘On a New Form of the Differential Thermometer;’ and ‘On the Permeability of Transparent Screens of extreme Tenuity by Radiant Heat,’ which led to his appointment, on the recommendation of Major Sabine, to the professorship of natural philosophy at the Royal Institution, where he delivered a course of probationary lectures in the spring of 1829. From this time he became a permanent resident in London, and was appointed professor of natural philosophy at the London university in 1832. In the following year he published a small introductory work, entitled, ‘Principles of Geometry Familiarly Illustrated,’ designed for the instruction of the young; and in 1836 he brought out another elementary work, under the name of ‘Principles of the Differential and Integral Calculus, applied to a variety of useful Purposes.’ He subsequently communicated to the Royal Society, of which he was elected a fellow, papers ‘On the Elasticity of Threads of Glass, and the Application of this Property to Torsion Balances;’ and also various experimental researches on the electric and chemical theories of galvanism, on electro-magnetism, and voltaic electricity. He died in the prime of life, September 15, 1837. Shortly before his death he was engaged in experiments, on an extensive scale, on the manufacture of glass for optical purposes, for the examination of the results of which a commission was appointed by the Government, with a view to their further prosecution by a public grant of money, or by affording increased facilities of experiment by a relaxation of the regulations of the excise.